Vertical transmission of group B streptococcus (GBS) from colonized mothers to their newborns can be attenuated by intrapartum antibiotic prophylaxis. The optimal duration of intrapartum antibiotic prophylaxis to prevent neonatal GBS colonization and sepsis is unknown. Penicillins enter the fetal bloodstream within minutes of administration to the mother and have been found to rapidly reach and maintain minimum inhibitory concentrations in the fetal bloodstream and amniotic fluid. Current protocols from the US Centers for Disease Control and Prevention recommend that intrapartum penicillin G prophylaxis be administered at least 4 hours prior to delivery. As many as 50% of women are not able to achieve 4 hours of intrapartum prophylaxis due to the rapidity of their labors. The ramifications of the 4-hour directive are widespread and include earlier admission of GBS colonized women to labor units, altered management of the labor course, invasive testing of newborns born less than 4 hours after initiation of prophylaxis, separation of these newborns from their mothers for observation protocols, increased parental and physician anxiety, and delayed discharge from the hospital setting. These interventions have not been proven to reduce or detect more cases of GBS sepsis in this group of infants. The possibility remains that shorter durations of exposure to intrapartum antibiotic prophylaxis are effective in preventing neonatal GBS colonization and sepsis, especially among low risk women who have intact amniotic membranes at the time of antibiotic initiation. The aim of this study is to determine if shorter durations (<4 hours) of intrapartum antibiotic prophylaxis are equivalent to e4 hours of therapy in a prospective cohort of maternal-infant dyads without other risk factors for early-onset GBS neonatal sepsis. Women colonized with GBS presenting in labor will undergo standard antibiotic prophylaxis. Penicillin G concentrations will be determined from umbilical cord blood samples at the time of delivery, and neonatal mucosal cultures will be performed at least 24 hours after delivery to detect GBS colonization, an accepted surrogate for risk of GBS sepsis. The relationship between duration of intrapartum penicillin prophylaxis, umbilical cord blood penicillin G levels, and neonatal colonization will be examined. Given increasing numbers of women receiving intrapartum antibiotic prophylaxis due to universal prenatal screening for GBS in the United States and the cost of intensified screening and surveillance of all infants exposed to <4 hours of intrapartum prophylaxis, we have a greater impetus to empirically identify the acceptable duration of prophylaxis for low risk GBS carriers. Project Narrative: The transmission of group B streptococcus from colonized women to their newborns during labor can be attenuated by treating women with intravenous antibiotic during labor. Current protocols from the CDC direct invasive testing and intensified surveillance of newborns born to women who were exposed to <4 hours of such treatment, despite the absence of evidence that this group of infants is at higher risk for GBS sepsis. The results of this study will be important in shaping early-onset GBS disease prevention protocols to minimize unnecessary interventions and to appropriately direct resources to those infants truly at risk for GBS disease.